The goal of this project is to understand the roles of electrolytes in cardiac function by quantitating their distribution among the anatomical compartments of heart muscle. This will be accomplished by electron microprobe X-ray analysis of tissue sections from rabbit papillary muscles. Emphasis is on the interrelationships among calcium, subcellular structures and contractility. Electron probe analysis is the only way to accurately and directly measure the ionic compartmentalization in intact, adult hearts. Studies previously carried out with fractionated components, skinned cells, and cultured tissue all suffer from questions of physiological integrity and relevance to the adult. Aims include: (a) Further validation and improvement of the microprobe method as applied to the heart. (b) Verification of calcium binding to in situ sarcolemma-glyco-calyx complex, and measurements to further define the role of this binding. (c) Measurement of calcium kinetics of in situ sarcoplasmic reticulum, in order to distinguish among hypotheses for its role(s) in excitation-contraction coupling. (d) Study of rearrangements of electrolyte compartmentalization that occur after physiological interventions involving cardiac glycosides, pH, phosphate, caffeine, and sodium. (e) Examination of subcellular electrolyte shifts at progressive time points in transient cardiac ischemia. (f) Measurement of calcium kinetics in the cytosol, and comparison with simultaneously occuring thermodynamic changes and with protein binding kinetics. Deeper understanding of cardiac function at the cellular and subcellular levels is essential for adequate understanding and progress in cardiac physiology, pathology, and pharmacology. Current understanding has emphasized the roles of electrolytes in controlling contractility, but hypothese about these roles are conflicting and often turn on questions of anatomical location of electrolyte pools under varying conditions. Our approach will help clarify these issues by providing the needed quantitative information about anatomical localization. The importance of this project is strengthened by the fact that it will be carried out in the framework of UCLA's Cardiovascular Research Laboratory, where its results can readily be correlated with those obtained by a wide variety of other investigative methods.